Cryogenic refrigerator and displacer

ABSTRACT

A cryogenic refrigerator includes a displacer including a body part and a heat conducting part, wherein the material of the heat conducting part has a higher thermal conductivity than the body part; a cylinder accommodating the displacer in such a manner as to allow the displacer to reciprocate in the axial directions of the cylinder, wherein an expansion space is formed between the cylinder and a low temperature end of the displacer; a clearance channel formed between the displacer and the cylinder so as to allow a refrigerant gas to flow into the expansion space; and a cooling stage positioned adjacent to the expansion space. The heat conducting part faces the cooling stage across the clearance channel.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority of theprior Japanese Patent Application No. 2012-001627, filed on Jan. 6,2012, the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a cryogenic refrigerator that producescryogenic temperatures by causing the Simon expansion using ahigh-pressure refrigerant gas fed from a compressor, and to a displacerused in the cryogenic refrigerator.

2. Description of the Related Art

For example, the cryogenic refrigerator described in Japanese Laid-OpenPatent Application No. 2011-17457 produces cold temperatures by causinga refrigerant gas in an expansion space to expand with the opening andclosing of a valve while causing a displacer to reciprocate inside acylinder. The refrigerant gas is fed into and discharged from theexpansion space through a clearance between the displacer and thecylinder. The refrigerant gas exchanges heat with a cooling stagepositioned on the peripheral side of the clearance and the expansionspace, so that the cold temperatures produced by the refrigerant gas inthe expansion space cool an object of cooling connected to the coolingstage.

SUMMARY OF THE INVENTION

According to an aspect of the present invention, a cryogenicrefrigerator includes a displacer including a body part and a heatconducting part, wherein a material of the heat conducting part has ahigher thermal conductivity than the body part; a cylinder accommodatingthe displacer in such a manner as to allow the displacer to reciprocatein axial directions of the cylinder, wherein an expansion space isformed between the cylinder and a low temperature end of the displacer;a clearance channel formed between the displacer and the cylinder so asto allow a refrigerant gas to flow into the expansion space; and acooling stage positioned adjacent to the expansion space, wherein theheat conducting part faces the cooling stage across the clearancechannel.

According to an aspect of the present invention, a displacer includes abody part; and a heat conducting part, wherein the heat conducting partis positioned at a low temperature end of the displacer, and a materialof the heat conducting part has a higher thermal conductivity than thebody part.

The object and advantages of the embodiments will be realized andattained by means of the elements and combinations particularly pointedout in the claims. It is to be understood that both the foregoinggeneral description and the following detailed description are exemplaryand explanatory and not restrictive of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects, features and advantages of the present invention willbecome more apparent from the following detailed description when readin conjunction with the accompanying drawings, in which:

FIG. 1 is a schematic diagram illustrating a cryogenic refrigerator anda displacer according to a first embodiment of the present invention;

FIG. 2 is a schematic diagram illustrating a cryogenic refrigerator anda displacer according to a second embodiment of the present invention;

FIG. 3 is a diagram illustrating a variation of a heat conducting partof the cryogenic refrigerator and the displacer according to the secondembodiment;

FIG. 4 is a schematic diagram illustrating another variation of the heatconducting part of the cryogenic refrigerator and the displaceraccording to the second embodiment; and

FIG. 5 is a schematic diagram illustrating a two-stage cryogenicrefrigerator according to a third embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

As described above, Japanese Laid-Open Patent Application No. 2011-17457describes a cryogenic refrigerator that produces cold temperatures bycausing a refrigerant gas in an expansion space to expand with theopening and closing of a valve while causing a displacer to reciprocateinside a cylinder. However, according to the technique described inJapanese Laid-Open Patent Application No. 2011-17457, the refrigerantgas that passes through the clearance exchanges heat only with thecooling stage positioned on the peripheral side of the clearance, thusmaking it difficult to ensure a sufficient substantial area for heatexchange.

According to an aspect of the present invention, a cryogenicrefrigerator and a displacer are provided that make it possible to moreeffectively ensure a sufficient substantial area for heat exchange.

In a cryogenic refrigerator and a displacer according to an aspect ofthe present invention, heat enters a heat conducting part from a coolingstage through a clearance channel on the periphery of the displacer. Theheat conducting part transfers heat to a refrigerant gas that enters theclearance channel because of expansion. As a result, the temperaturedifference of the cooling stage is reduced, and a substantial heatexchange area that contributes to heat exchange increases, so that it ispossible to improve heat exchange efficiency.

A description is given below, with reference to the accompanyingdrawings, of embodiments of the present invention.

[a] First Embodiment

A cryogenic refrigerator 1 according to a first embodiment is, forexample, a Gifford-McMahon (GM) refrigerator that uses helium gas as arefrigerant gas. The cryogenic refrigerator 1 includes a displacer 2, acylinder 4, and a cooling stage 5 that has a bottomed cylinder (tube)shape. A clearance C (a clearance channel) and an expansion space 3 areformed between the displacer 2 and the cylinder 4. The cooling stage 5is adjacent to and encloses the expansion space 3. The displacer 2includes a body part 2 a and a heat conducting part 2 b. The heatconducting part 2 b is formed of a material that has a higher thermalconductivity than the body part 2 a. The heat conducting part 2 b facesthe cooling stage 5 across the clearance C. The cooling stage 5 isformed of, for example, copper, aluminum, stainless steel or the like.

Here, the heat conducting part 2 b has a lower coefficient of thermalexpansion than the body part 2 a. The heat conducting part 2 b includesan overlapping part 2 ba that overlaps the body part 2 a in thedirections of strokes (stroke directions) of the displacer 2 (in whichthe displacer 2 reciprocates). The body part 2 a includes an overlappedpart 2 a b that corresponds to the overlapping part 2 ba. In the firstembodiment, the heat conducting part 2 b has a two-stage (stepped)columnar shape, and the overlapping part 2 ba is formed by the second(upper) columnar shape from the bottom in FIG. 1.

The overlapping part 2 ba includes first hole parts 2 bah, and theoverlapped part 2 ab includes second hole parts 2 abh that correspond tothe first hole parts 2 bah. The body part 2 a and the heat conductingpart 2 b are connected by press-fit pins 6 (an insertion member) thatare press-fit and inserted into both the second hole parts 2 abh and thefirst hole parts 2 bah. The press-fit pins 6 are provided at suitablepoints in a circumferential direction of the displacer 2. A materialthat has a higher thermal conductivity than at least the body part 2 a,such as copper, aluminum, stainless steel or the like, is used for theheat conducting part 2 b. The press-fit pins 6 may be either Bakelite(phenol containing cloth) or stainless steel. The overlapping part 2 bais fixed to the overlapped part 2 ab by press-fitting the press-fit pins6 into the first hole parts 2 bah and the second hole parts 2 abh.

The cylinder 4 accommodates the displacer 2 in such a manner as to allowthe displacer 2 to reciprocate in the longitudinal directions of thecylinder 4. For example, stainless steel is used for the cylinder 4 interms of strength, thermal conductivity, helium blocking capability,etc.

A Scotch yoke mechanism (not graphically illustrated) that causes thedisplacer 2 to reciprocate is provided at a high-temperature end of thedisplacer 2, so that the displacer 2 reciprocates along the axialdirections of the cylinder 4.

The displacer 2 has a cylindrical peripheral (exterior circumferential)surface. The displacer 2 is filled with a regenerator material. Theinternal space of the displacer 2 forms a regenerator 7. An upper flowrectifier 9 that rectifies (regulates) a flow of helium gas is providedon the upper end side, that is, the room temperature chamber 8 side, ofthe regenerator 7. A lower flow rectifier 10 is provided on the lowerend side of the regenerator 7.

An opening 11 through which a refrigerant gas flows from a roomtemperature chamber 8 into the displacer 2 is formed at the hightemperature end of the displacer 2. The room temperature chamber 8,which is a space defined by the cylinder 4 and the high temperature endof the displacer 2, changes in volume with the reciprocation of thedisplacer 2.

Of the various pipes that interconnect a compressor 12, a supply valve13, and a return valve 14, which form a intake and outlet system, a pipecommon to supply and discharge (a supply and discharge common pipe) isconnected to the room temperature chamber 8. Further, a seal 15 isattached between part of the displacer 2 on the high temperature endside and the cylinder 4.

Openings 16 that introduce a refrigerant gas into the expansion space 3via the clearance C are formed at a low temperature end of the displacer2. The expansion space 3, which is a space defined by the cylinder 4 andthe displacer 2, changes in volume with the reciprocation of thedisplacer 2. The cooling stage 5, which is thermally coupled to anobject of cooling, is placed at a position corresponding to theexpansion space 3 on the periphery and the bottom of the cylinder 4. Thecooling stage 5 is cooled by a refrigerant gas that passes through theclearance C.

For example, Bakelite (phenol containing cloth) is used for the bodypart 2 a of the displacer 2 in view of specific gravity, strength,thermal conductivity, etc. The regenerator material is formed of, forexample, a wire mesh. FIG. 1 illustrates the cryogenic refrigerator 1 inoperation. Therefore, because of slight contraction of the body part 2 adue to low temperature, the body part 2 a and the heat conducting part 2b are equal in outside diameter. However, at normal temperature (5° C.to 35° C.), the outside diameter of the heat conducting part 2 b isslightly smaller than the outside diameter of the body part 2 a.

Next, a description is given of an operation of the cryogenicrefrigerator 1. At some point of time during a refrigerant gas supplyprocess, the displacer 2 is positioned at its bottom dead center insidethe cylinder 4. When the supply valve 13 is opened at the same time withor slightly before or after that point of time, high-pressure helium gasis supplied into the cylinder 4 through the supply valve 13 and thesupply and discharge common pipe, and flows into the regenerator 7inside the displacer 2 through the opening 11 positioned at the top(high temperature end) of the displacer 2. The high-pressure helium gasthat has flown into the regenerator 7 is supplied into the expansionspace 3 through the openings 16, positioned in a lower part of thedisplacer 2, and the clearance C while being cooled by the regeneratormaterial.

Thus, the expansion space 3 is filled with the high-pressure helium gas,and the supply valve 13 is closed. At this point, the displacer 2 ispositioned at its top dead center inside the cylinder 4. When the returnvalve 14 is opened at the same time with or slightly before or afterthat point, the pressure of the helium (refrigerant) gas inside theexpansion space 3 is reduced, so that the helium gas expands. The heliumgas inside the expansion space 3, whose temperature has been loweredbecause of its expansion, absorbs the heat of the cooling stage 5through the clearance C.

The displacer 2 moves toward the bottom dead center, so that the volumeof the expansion space 3 is reduced. The helium gas inside the expansionspace 3 is returned to the intake side of the compressor 12 through theclearance C, the openings 16, the regenerator 7, and the opening 11.During this process, the regenerator material is cooled by therefrigerant gas (helium gas). Letting this process be one cycle, thecryogenic refrigerator 1 cools the cooling stage 5 by repeating thiscooling cycle.

According to the cryogenic refrigerator 1 and the displacer 2 of thefirst embodiment, the heat conducting part 2 b constantly faces thecooling stage 5 across the clearance C. Heat that enters from thecooling stage 5 also enters the heat conducting part 2 b via the heliumgas in the clearance C. Therefore, when the low-temperature helium gasgenerated in the expansion space 3 passes through the clearance C, heatexchange is performed not only between the helium gas and the coolingstage 5 but also between the helium gas and the heat conducting part 2b. As a result, it is possible to increase a substantial area for heatexchange (heat exchange area) between the cooling stage 5 and thelow-temperature helium gas.

Further, the heat that has entered the heat conducting part 2 b isfurther transferred through the inside of the heat conducting part 2 btoward the expansion space 3. Therefore, it is possible to furtherimprove heat exchange efficiency by configuring the heat conducting part2 b so that the heat conducting part 2 b comes into contact with thelow-temperature helium gas inside the expansion space 3.

In contrast, according to a configuration without the heat conductingpart 2 b, that is, according to the conventional displacer where a partcorresponding to the heat conducting part 2 b is formed of Bakelite, theheat exchange between helium gas and Bakelite is so limited that nosubstantial heat exchange is performed. Therefore, when low-temperaturehelium gas generated in an expansion space passes through a clearance,heat exchange is performed only between the helium gas and a coolingstage.

Thus, according to the cryogenic refrigerator 1 and the displacer 2 ofthe first embodiment, it is possible to cause the heat conducting part 2b as well to effectively contribute to heat exchange, compared with theconventional displacer, so that it is possible to increase a substantialheat exchange area. Further, the above-described flow (transfer) of heatgenerated inside the heat conducting part 2 b makes it possible tofurther improve heat exchange efficiency. That is, it is possible toreduce the temperature difference of the cooling stage 5 in a verticaldirection in FIG. 1. In particular, it is possible to reduce thetemperature difference in the case of providing an object of coolingbelow the cooling stage 5.

Further, when Bakelite is used for a part corresponding to the heatconducting part 2 b as in the conventional displacer, the part contractswith a decrease in temperature because of a relatively high coefficientof thermal expansion of Bakelite, so that a part corresponding to theoverlapping part 2 ba may come off a part corresponding to theoverlapped part 2 ab. In contrast, according the cryogenic refrigerator1 and the displacer 2 of the first embodiment, the overlapping part 2ba, which has a lower coefficient of thermal expansion than the bodypart 2 a, is provided inside (on the inner circumference side of) theoverlapped part 2 ab of the body part 2 a. Therefore, when theoverlapped part 2 ab of the body part 2 a is cooled to contract, asqueezing force is exerted on the overlapping part 2 ba of the heatconducting part 2 b, so that it is possible to prevent the overlappingpart 2 ba from coming off the overlapped part 2 ab.

Further, according to the cryogenic refrigerator 1 and the displacer 2of the first embodiment, the heat conducting part 2 b also contributesto heat exchange, thereby increasing a substantial heat exchange area.Therefore, even when the cooling stage 5 and the clearance C are reducedin length in the axial directions (the moving directions of thedisplacer 2) compared with the conventional displacer, it is possible toobtain a desired refrigerating capacity. As a result, it is possible toreduce channel resistance and pressure loss in the clearance C, so thatit is possible to increase the refrigeration efficiency of the cryogenicrefrigerator 1. Further, reduction in the volume of the clearance Cleads to a decrease in dead volume that does not contribute togeneration of cold temperatures. This may be expected to reduce apressure difference between a high pressure and a low pressure duringone cycle due to dead volume.

The overlapping part 2 ba and the overlapped part 2 ab may form a screwpart so as to be screwed to each other. For example, the overlappingpart 2 ba and the overlapped part 2 ab may be screwed to each other withtheir respective threaded parts mating with each other. This allows thebody part 2 a and the heat conducting part 2 b to be more easilyattached to and detached from each other. In this case as well, when theoverlapped part 2 ab of the body part 2 a is cooled to contract, asqueezing force is exerted on the overlapping part 2 ba of the heatconducting part 2 b. Thus, it is possible to further prevent theoverlapping part 2 ba from coming off the overlapped part 2 ab.

[b] Second Embodiment

In the above-described first embodiment, the heat conducting part 2 bhas a columnar shape, while the heat conducting part 2 b may have atubular shape as described below. FIG. 2 is a schematic diagramillustrating a cryogenic refrigerator 21 and a displacer 22 according toa second embodiment. In FIG. 2, the same elements as those of the firstembodiment of FIG. 1 are referred to by the same reference numerals, anda description is basically given of differences from the firstembodiment.

According to the second embodiment, the displacer 22 includes a bodypart 22 a and a heat conducting part 22 b. The heat conducting part 22 bhas a tubular shape, and the entire heat conducting part 22 b forms anoverlapping part 22 ba that overlaps the body part 22 a in the strokedirections of the displacer 22. A portion of the body part 22 a that ispositioned on the low temperature side of the openings 16 (that is,below the openings 16 in FIG. 2) has a smaller diameter than a portionof the body part 22 a that is positioned on the high temperature side ofthe openings 16 (that is, above the openings 16 in FIG. 2). Thissmaller-diameter portion of the body part 22 a forms an overlapped part22 ab that corresponds to the overlapping part 22 ba.

The overlapping part 22 ba includes first hole parts 22 bah, and theoverlapped part 22 ab includes second hole parts 22 abh that correspondto the first hole parts 22 bah. The body part 22 a and the heatconducting part 22 b are connected by press-fit pins 26 (an insertionmember) that are press-fit and inserted into both the second hole parts22 abh and the first hole parts 22 bah. Like in the first embodiment, amaterial that has a higher thermal conductivity than at least the bodypart 22 a, such as copper, aluminum, stainless steel or the like, isused for the heat conducting part 22 b. In this embodiment as well, thepress-fit pins 26 may be either Bakelite (phenol containing cloth) orstainless steel. The overlapping part 22 ba is fixed to the overlappedpart 22 ab by inserting the press-fit pins 26 into the first hole parts22 bah and the second hole parts 22 abh.

According to the cryogenic refrigerator 21 and the displacer 22 of thesecond embodiment as well, it is possible to increase a heat exchangearea by causing the heat conducting part 22 b to contribute to heatexchange as in the first embodiment. In addition, in the secondembodiment, the heat conducting part 22 b is placed only on theperipheral (outer circumferential) side of the displacer 22, whichcontributes to heat exchange compared with the first embodiment.Therefore, it is possible to reduce the volume and mass of the heatconducting part 22 b compared with the volume and mass of the heatconducting part 2 b of the first embodiment, and thus to reduce the massof the entire displacer 22, which is a movable part, compared with themass of the displacer 2 of the first embodiment.

Further, the reciprocation of the heat conducting part 22 b, which is aconductor, under the presence of a magnetic field generates eddycurrent, which causes heat generation, that is, copper loss. Accordingto the second embodiment, the volume of the heat conducting part 22 b isrelatively small. Therefore, it is possible to control generation ofcopper loss accordingly.

Further, the heat conducting part 22 b may be formed of a standardizedtubular material. Therefore, it is possible to reduce cost compared withthe first embodiment.

As described above, the generation of copper loss is expected to becontrolled by reducing the volume of a conductor, while the generationof copper loss may also be controlled by controlling generation of eddycurrent by designing a shape. For example, FIG. 3 illustrates avariation of the tubular heat conducting part 22 b illustrated in FIG.2, where a slit S is formed to make the heat conducting part 22 bdiscontinuous in its circumferential directions. In FIG. 3, (a) is aplan view and (b) is a side view of the variation of the heat conductingpart 22 b. According to this configuration, it is possible to preventeddy current from continuously flowing in the circumferential directionsin particular, so that it is possible to control generation of copperloss more effectively.

Further, FIG. 4 illustrates another variation of the heat conductingpart 22 b. As illustrated in FIG. 4, the heat conducting part 22 b mayhave a bottomed tube shape. The heat conducting part 22 b of a bottomedtube shape causes heat that has entered the heat conducting part 22 bfrom the cooling stage 5 to be exchanged between a bottom part 22 b b ofthe heat conducting part 22 b and the expansion space 3. As a result, itis possible to increase cooling efficiency compared with the cryogenicrefrigerator 21 of FIG. 2.

[c] Third Embodiment

In the above-described first and second embodiments, single-stagerefrigerators are illustrated, while an embodiment of the presentinvention may also be applied to a two-stage refrigerator as describedbelow. FIG. 5 is a schematic diagram illustrating a cryogenicrefrigerator 31 and first and second displacers 32 and 36 according to athird embodiment.

Like the cryogenic refrigerators 1 and 21 of the first and secondembodiments, the cryogenic refrigerator 31 according to the thirdembodiment is a Gifford-McMahon (GM) refrigerator using helium gas as arefrigerant gas. As illustrated in FIG. 5, the cryogenic refrigerator 31includes the first displacer 32 and the second displacer 36. The seconddisplacer 36 is connected to the first displacer 32 in a longitudinaldirection of the second displacer 36. As illustrated in FIG. 5, thefirst displacer 32 and the second displacer 36 are connected via, forexample, a pin 33, a connector 34, and a pin 35.

A first cylinder 37 and a second cylinder 38 are formed as a unit. A lowtemperature end of the first cylinder 37 and a high temperature end ofthe second cylinder 38 are connected at the bottom of the first cylinder37. The second cylinder 38 is coaxial with the first cylinder 37, and isa cylindrical member that has a smaller diameter than the first cylinder37. The first cylinder 37 accommodates the first displacer 32 in such amanner as to allow the first displacer 32 to reciprocate in thelongitudinal directions of the first cylinder 37. The second cylinder 38accommodates the second displacer 36 in such a manner as to allow thesecond displacer 36 to reciprocate in the longitudinal directions of thesecond cylinder 38.

For example, stainless steel is used for the first cylinder 37 and thesecond cylinder 38 in consideration of strength, thermal conductivity,helium blocking capability, etc. The second displacer 36 has a coatingof an abrasion resistant resin such as fluororesin on the peripheral(exterior circumferential) surface of its metallic cylinder of, forexample, stainless steel.

A Scotch yoke mechanism (not graphically illustrated) that causes thefirst displacer 32 and the second displacer 36 to reciprocate isprovided at a high-temperature end of the first cylinder 37, so that thefirst displacer 32 and the second displacer 36 reciprocate along thefirst cylinder 37 and the second cylinder 38, respectively.

The first displacer 32 has a cylindrical peripheral (exteriorcircumferential) surface. The first displacer 32 is filled with a firstregenerator material. The internal space of the first displacer 32serves as a first regenerator 39. A flow rectifier 40 and a flowrectifier 41 are provided on and under the first regenerator 39. A firstopening 42 through which a refrigerant gas flows from a room temperaturechamber 69 into the first displacer 32 is formed at a high temperatureend of the first displacer 32. The room temperature chamber 69, which isa space defined by the first cylinder 37 and the high temperature end ofthe first displacer 32, changes in volume with the reciprocation of thefirst displacer 32. Of the pipes that interconnect a compressor 43, asupply valve 44, and a return valve 45, which form an intake and outletsystem, a pipe common to supply and discharge (a supply and dischargecommon pipe) is connected to the room temperature chamber 69. Further, aseal 46 is attached between part of the first displacer 32 on the hightemperature end side and the first cylinder 37.

Second openings 48 that introduce a refrigerant gas into a firstexpansion space 47 via a first clearance C1 (a clearance channel) areformed at a low temperature end of the first displacer 32. The firstexpansion space 47, which is a space defined by the first cylinder 37and the first displacer 32, changes in volume with the reciprocation ofthe first displacer 32. A first cooling stage 49, which is thermallycoupled to an object of cooling (not graphically illustrated), is placedat a position corresponding to the first expansion space 47 on theperiphery of the first cylinder 37. The first cooling stage 49 is cooledby a refrigerant gas that passes through the first clearance C1.

The second displacer 36 has a cylindrical peripheral (exteriorcircumferential) surface. The second displacer 36 is filled with asecond regenerator material. The internal space of the second displacer36 serves as a second regenerator 50. The first expansion space 47 and ahigh temperature end of the second displacer 36 are connected by acommunicating passage (not graphically illustrated). A refrigerant gasflows from the first expansion space 47 into the second regenerator 50through this communicating passage.

A third opening 52 that introduces a refrigerant gas into a secondexpansion space 51 via a second clearance C2 (a clearance channel) isformed at a low temperature end of the second displacer 36. The secondexpansion space 51, which is a space defined by the second cylinder 38and the second displacer 36, changes in volume with the reciprocation ofthe second displacer 36. The second clearance C2 is defined by a lowtemperature end part of the second cylinder 38 and the second displacer36. The second clearance C2 is larger than a clearance between thesecond displacer 36 having a helical groove 63 as described below andthe second cylinder 38.

A second cooling stage 53, which is thermally coupled to an object ofcooling (not graphically illustrated), is placed at a positioncorresponding to the second expansion space 51 on the periphery of thesecond cylinder 38. The second cooling stage 53 is cooled by arefrigerant gas that passes through the second clearance C2.

The first displacer 32 includes a body part 32 a and a heat conductingpart 32 b. The heat conducting part 32 b is formed of a material thathas a higher thermal conductivity than the body part 32 a. For example,Bakelite (phenol containing cloth) is used for the body part 32 a of thefirst displacer 32 in view of specific gravity, strength, thermalconductivity, etc. A material that has a higher thermal conductivitythan at least the body part 32 a, such as copper, aluminum, stainlesssteel or the like, is used for the heat conducting part 32 b.

The heat conducting part 32 b has a lower coefficient of thermalexpansion than the body part 32 a. The heat conducting part 32 bincludes an overlapping part 32 ba that overlaps the body part 32 a inthe directions of strokes (stroke directions) of the first displacer 32.The body part 32 a includes an overlapped part 32 ab that corresponds tothe overlapping part 32 ba.

The second displacer 36 includes a body part 36 a and a heat conductingpart 36 b. The heat conducting part 36 b is formed of a material thathas a higher thermal conductivity than the body part 36 a. For example,Bakelite (phenol containing cloth) is used for the body part 36 a of thesecond displacer 36 in view of specific gravity, strength, thermalconductivity, etc. A material that has a higher thermal conductivitythan at least the body part 36 a, such as copper, aluminum, stainlesssteel or the like, is used for the heat conducting part 36 b.

The heat conducting part 36 b has a lower coefficient of thermalexpansion than the body part 36 a. The heat conducting part 36 bincludes an overlapping part 36 ba that overlaps the body part 36 a inthe directions of strokes (stroke directions) of the second displacer36. The body part 36 a includes an overlapped part 36 ab thatcorresponds to the overlapping part 36 ba.

The first regenerator material is formed of, for example, a wire mesh.The second regenerator material is formed by holding a regeneratormaterial such as lead spheres with felt and a wire mesh in the axialdirections.

The helical groove 63 is formed on the peripheral (exteriorcircumferential) surface of the second displacer 36. The helical groove36 has a starting end communicating with the second expansion space 51through the second clearance C2, and helically extends toward the firstexpansion space 47.

According to the third embodiment as well, the first displacer 32 andthe second displacer 36 include the heat conducting part 32 b and theheat conducting part 36 b at their respective cold (low) temperatureends. Both the heat conducting part 32 b and the heat conducting part 36b have a two-stage (stepped) columnar shape. The heat conducting part 32b is fixed to the body part 32 a with press-fit pins 54. The heatconducting part 36 b is fixed to the body part 36 a with press-fit pins55. According to the third embodiment as well, for the reasons statedabove with respect to the first and the second embodiment, it ispossible to improve cooling efficiency by increasing a substantial heatexchange area for each of the first cooling stage 49 and the secondcooling stage 53.

All examples and conditional language provided herein are intended forpedagogical purposes of aiding the reader in understanding the inventionand the concepts contributed by the inventors to further the art, andare not to be construed as limitations to such specifically recitedexamples and conditions, nor does the organization of such examples inthe specification relate to a showing of the superiority or inferiorityof the invention. Although one or more embodiments of the presentinvention have been described in detail, it should be understood thatthe various changes, substitutions, and alterations could be made heretowithout departing from the spirit and scope of the invention.

For example, in the above-described embodiments, a description is givenof the case where the number of stages of a cryogenic refrigerator isone or two. However, the number of stages may be suitably selected, andmay be, for example, three. Further, in the above-described embodiments,a description is given of the case where the cryogenic refrigerator is aGM refrigerator. However, embodiments of the present invention are notlimited to the GM refrigerator, and may be applied to any refrigeratorshaving a displacer, such as Stirling refrigerators and Solvay cyclerefrigerators.

According to an aspect of the present invention, it is possible toimprove the refrigeration efficiency of a cryogenic refrigerator byimproving its heat exchange efficiency by effectively increasing a heatexchange area that substantially contributes to heat exchange through aside clearance without increasing the length of a cooling stage in theaxial directions of the cryogenic refrigerator. Accordingly, embodimentsof the present invention may be applied to various kinds of cryogenicrefrigerators.

What is claimed is:
 1. A cryogenic refrigerator, comprising: a displacerincluding a body part and a heat conducting part, wherein a material ofthe heat conducting part has a higher thermal conductivity than the bodypart; a cylinder accommodating the displacer in such a manner as toallow the displacer to reciprocate in axial directions of the cylinder,wherein an expansion space is formed between the cylinder and a lowtemperature end of the displacer; a clearance channel formed between thedisplacer and the cylinder so as to allow a refrigerant gas to flow intothe expansion space; and a cooling stage positioned adjacent to theexpansion space, wherein the heat conducting part faces the coolingstage across the clearance channel.
 2. The cryogenic refrigerator asclaimed in claim 1, wherein the heat conducting part has a lowercoefficient of thermal expansion than the body part.
 3. The cryogenicrefrigerator as claimed in claim 1, wherein the heat conducting partincludes an overlapping part that overlaps the body part in strokedirections of the displacer, and the body part includes an overlappedpart corresponding to the overlapping part.
 4. The cryogenicrefrigerator as claimed in claim 3, wherein the heat conducting part hasa bottomed tube shape.
 5. The cryogenic refrigerator as claimed in claim3, wherein the overlapping part and the overlapped part form a screwpart.
 6. The cryogenic refrigerator as claimed in claim 3, furthercomprising: an insertion member inserted into a first hole part formedin the overlapping part and a second hole part formed in the overlappedpart so as to connect the body part and the heat conducting part.
 7. Thecryogenic refrigerator as claimed in claim 1, wherein the heatconducting part has a tubular shape with a slit that makes the heatconducting part discontinuous in a circumferential direction thereof. 8.The cryogenic refrigerator as claimed in claim 1, wherein the materialof the heat conducting part is selected from the group consisting ofcopper, aluminum, and stainless steel.
 9. A displacer, comprising: abody part; and a heat conducting part, wherein the heat conducting partis positioned at a low temperature end of the displacer, and a materialof the heat conducting part has a higher thermal conductivity than thebody part.
 10. The displacer as claimed in claim 9, wherein an outsidediameter of the heat conducting part is smaller than an outside diameterof the body part at normal temperature.